The combustion of coal and other fossil fuels during the production of steam or power produces combustion deposits, i.e., slag, ash and/or soot, that accumulate on the surfaces in the boiler. These deposits generally decrease the efficiency of the boiler, particularly by reducing heat transfer in the boiler. When combustion deposits accumulate on the heat transfer tubes that transfer the energy from the combustion to water, creating steam, for example, the heat transfer efficiency of the tubes decreases, which in turn decreases the boiler efficiency. To maintain a high level of boiler efficiency, the boiler surfaces are periodically cleaned. These deposits are periodically removed by directing a cleaning medium, e.g., air, steam, water, or mixtures thereof, against the surfaces upon which the deposits have accumulated at a high pressure or high thermal gradient with cleaning devices known generally in the art as sootblowers. Sootblowers may be directed to a number of desired points in the boiler, including the heat transfer tubes.
To avoid or eliminate completely the negative effects of combustion deposits on boiler efficiency, the boiler surfaces and, in particular, the heat transfer tubes, would need to be essentially free of deposits at all times. Maintaining this level of cleanliness would require virtually continuous cleaning. Maintaining completely soot-free boilers is not practical under actual operating conditions because the cleaning itself is expensive and creates wear and tear on the boiler system. Cleaning generally requires diverting energy generated in the boiler, which negatively impacts the efficiency of the boiler and makes the cleaning costly. Injection of the cleaning medium into the boiler also reduces the efficiency of the boiler and prematurely damages heat transfer surfaces in the boiler, particularly if they are over-cleaned. Boiler surfaces, including heat transfer tubes, can also be damaged as a result of erosion by high velocity air or steam jets and/or as a result of thermal impact from jets of a relatively cool cleaning medium, especially air or liquid, impinging onto the hot boiler surfaces, especially if they are relatively clean. Boiler surface and water wall damage resulting from sootblowing is particularly costly because correction requires boiler shutdown, cessation of power production, and immediate attention that cannot wait for scheduled plant outages. Therefore, it is important that these surfaces not be cleaned unnecessarily or excessively.
The goal of maximizing boiler cleanliness is balanced against the costs of cleaning in order to improve boiler efficiency and, ultimately, boiler performance. Accordingly, reasonable, but less than ideal, boiler cleanliness levels are typically maintained in the boiler. Sootblower operation is regulated to maintain those selected cleanliness levels in the boiler. Different areas of the boiler may accumulate deposits at different rates and require different levels of cleanliness and different amounts of cleaning to attain a particular level of cleanliness. A boiler may be characterized by one or more heat zones, each heat zone having its heat transfer efficiency and cleanliness level measured and set individually. A boiler may contain, for example, 35 or even 50 heat zones. It is important that these cleanliness levels be coordinated in order to satisfy the desired boiler performance goals. A heat zone may include one or more sootblowers, as well as one or more sensors.
Sootblowers may operate subject to a number of parameters that determine how the sootblower directs a fluid against a surface, including jet progression rate, rotational speed, spray pattern, fluid velocity, media cleaning pattern, and fluid temperature and pressure. The combination of settings for these parameters that is applied to a particular sootblower determines its cleaning efficiency. These settings can be varied to change the cleaning efficiency of the sootblower. The cleaning efficiency of the sootblowers can be manipulated to maintain the desired cleanliness levels in the boiler. In addition, the frequency of operation of sootblowers can be determined according to different methods. For example, sootblowers can be operated on a time schedule based on past experience, or on measured boiler conditions, such as changes in the heat transfer rate of the heat transfer tubes. Boiler conditions may be determined by visual observation, by measuring boiler parameters, or by the use of sensors on the boiler surfaces to measure conditions indicative of the level of soot accumulation, e.g., heat transfer rate degradation of the heat transfer tubes.
One type of known system is designed to maintain a predefined cleanliness level by controlling the sootblower operating parameters for one or more sootblowers. After the sootblower is operated to clean a surface, one or more sensors are used to measure the heat transfer improvement resulting from the cleaning operation, and determine the effectiveness of the immediately preceding sootblowing operation in cleaning the surface. The measured cleanliness data is compared against the predefined cleanliness standard that is stored in the processor. One or more sootblower operating parameters can be adjusted to alter the aggressiveness of the next sootblowing operation based on the relative effectiveness of the previous sootblowing operation and the boiler operating conditions. The goal is to maintain the required level of heat transfer surface cleanliness for the current boiler operating conditions while minimizing the detrimental effects of sootblowing. The general boiler operating conditions may be determined by factors such as fuel/air mixtures, feed rates, and the type of fuel used. Given the operating conditions, the system determines the sootblower operating parameters that can be used to approximate the required level of heat transfer surface cleanliness, using a database of historical boiler operating conditions and their corresponding operating parameters as a starting point.
Boiler operation is generally governed by one or more boiler performance goals. Boiler performance is generally characterized in terms of heat rate, capacity, net profit, and emissions (e.g., NOx, CO), as well as other parameters. One principle underlying the cleaning operation is to maintain the boiler performance goals. The above-described system does not relate the boiler performance to the required level of heat surface cleanliness and, therefore, to the optimum operating parameters. The system assumes that the optimal soot level efficiency set point, i.e., the required level of heat surface cleanliness, is given: it may be entered by an operator, for example. Accordingly, the system assumes that required cleanliness levels for desired boiler performance goals are determined separately and provides no mechanism for selecting cleanliness levels for individual heat zones, for coordinating the cleanliness levels for different heat zones in a boiler, for coordinating sootblower parameters according to different cleanliness levels, i.e., in different heat zones, or for coordinating the cleanliness levels as a function of the boiler performance objectives, in terms of the boiler outputs. Accordingly, although achieving boiler performance targets is a primary objective in operating a boiler, the sootblower operating settings are not related to the boiler performance targets in the prior art system.
As discussed above, because different parts of a boiler may require different amounts of monitoring and cleaning, a boiler is typically divided into one or more heat zones, each of which may be set to a different cleanliness level. The required cleanliness levels for the different heat zones in a boiler should be carefully selected and coordinated to achieve particular boiler performance goals. Not only can performance goals change, but selecting performance goals does not necessarily determine the efficiency set points for the sootblowers in the system. The desired cleanliness levels for desired performance targets are not necessarily known beforehand. The efficiency set points of the sootblowers that are necessary to achieve a given set of performance values may vary, for example, according to the operating conditions of the boiler. In addition, the sootblower operating settings that are useful to achieve a given set of performance values are not necessarily known beforehand and will also vary according to the operating conditions of the boiler and other factors. A need exists for a method and system for determining cleanliness levels and/or sootblower operating parameters using boiler performance targets. A need exists for a method and system for determining and coordinating a complete set of cleanliness factors for the heat zones in a boiler using boiler performance targets.